Oriented polymer strap

ABSTRACT

Manufacture of strapping from resin at least 75 percent of which is polyethylene having a density of at least 0.95 g/cc and preferably from 0.955 to 0.967 g/cc and a melt index of from 0.2 to 1.0 and preferably from 0.4 to 0.9 by extruding a billet, roll orienting the billet, followed by stretching at ambient temperature, followed by stretching in a liquid bath maintained at from 93*C. to the melting point of the polyethylene and preferably from 110* to 130*C. to give a total deformation of from 10 to 18 times and preferably 11 to 13.5 times.

United States Patent Starkweather, Jr.

[ Mar. 26, 1974 Related US. Application Data [62] Division of Ser.No.'845,692, July 29, 1969, Pat. No.

[52] US. Cl. 260/897 B, 260/949 GD, 264/178,

264/210, 264/280, 264/288 [51] Int. Cl. C08f 29/12 [58] Field of Search260/897; 264/210, 288

[56] References Cited UNITED STATES PATENTS 3/1972 Starkweather 260/89711/1967 Dunnington et a1. 161/165 3,290,420 12/1966 Orser 264/2103,104,937 9/1963 Wyckoff 18/48 FOREIGN PATENTS OR APPLICATIONS 1,002,3738/1965 Great Britain 260/897 Primary Examiner-Murray Tillman AssistantExaminerC. J. Seccuro [5 7] ABSTRACT Manufacture of strapping from resinat least percent of which is polyethylene having a density of at least0.95 g/cc and preferably from 0.955 to 0.967 g/cc and a melt index offrom 0.2 to 1.0 and preferably from 0.4 to 0.9 by extruding a billet,roll orienting the billet, followed by stretching at ambienttemperature, followed by stretching in a liquid bath maintained at from93C. to the melting point of the polyethylene and preferably from to C.to give a total deformation of from 10 to 18 times and preferably 11 totimes.

3 Claims, 1 Drawing Figure ORIENTED POLYMER STRAP CROSS-REFERENCE TORELATED APPLICATION This application is a divisional of copending US.Pat. application Ser. No. 845,692, filed July 29, 1969 by Howard W.Starkweather, Jr now US. Pat. 3,651,196.

BACKGROUND OF THE INVENTION The conventional strapping used to bindlarge containers is currently made from steel. Another type of strappingwhich is widely used where the very high strengths of steel areunnecessary is a strap formed of a series of parallel rayon cords heldtogether by means of a binder. This lattertype of strapping is describedin U. S. Pat. No. 3,028,281, issued Apr; 3, 1962 to Thomas J. Karass.Another type of strapping is roll oriented plastic strapping such asdescribed in U. S. Patent 3,354,023 issued Nov. 21, 1967 to Gordon BealeDunnington and Reuben Thomas Fields on Nov. 21, 1967.

SUMMARY OF THE lNVENTlON This invention has as an object the manufactureof a strap from polyethylene which has a strength high enough to act asa substitute for steel strapping or roll oriented strapping from otherpolymers.

These objects are accomplished by the following invention in whichpolyethylene having a density of at least 0.95 and preferably from 0.955to 0.967 and a melt index of from 0.2 to 1.0 and preferably from 0.4 to0.9 is extruded into a billet which is then rolled from 2.5 to 6.5 timesits original length so as to produce a uniplanar, axial orientedcrystalline product which strap is then stretched further at ambienttemperature from 1.05 to 3.0 times its length which strap then isstretched further in a hot oil bath from 1.6 to 3.0 times and preferablyfrom 2.0 to 2.5 times its length to produce a strapping having adeformation ratio of m 18 times andpreferably from 1 l to 13.5 times theoriginal length of the billet. The oil bath may be maintained ata'temperature required to heat the strapping being hot stretched at from93C. up to the melting point of the polyethylene with the range of from110 to.130C. being preferred. The residence time in the oil bath and thetemperature ofthe oil bath are interdependent variables in controllingthis temperature. The width of the final strapping is preferably from0.4 to 0.75 times the width of the billet from which it is rolled. Toaccomplish this objective it has been found that the uniformity of theextruded billet prior to the roll orienting step is of extremeimportance to the sucessful production of a high strength rolled shape.This uniformity relates both to the cross-sectional dimensions of theextruded billet and to any orientation imposed on the billet. Anirregular billet cannot be roll oriented into a useful hgih strengthstrapping because some sections will pass their maximum orientationpotential and fibrillate or become hairy before the central sectionshave been oriented to their optimum. Roll-oriented polyethylene tapesand ribbons have been made before but such prior art tapes and ribbonshave not had sufficient strength to compete with steel strapping orother polymer strapping because it had not been possible to impartsufficient orientation to such polyethylene tapes and ribbons for themto have the requisite strength. The strapping of this invention ispreferably from 4 to 50 mils thick and from A to /1 of an inch widealthough wider widths can be made and are desirable for some purposessuch as helically wrapping large diameter pipe, and widths as narrow asA; inch are useful. The strapping of this invention has particularutility as a tape with an adhesive on one side thereof for use inpackaging moderate sized objects due to the dead bend" property of thetape without significant loss in tensile strength. That is, the strapsor tapes of the present invention may be creased to or even without anysignificant tendency to straighten and without significant loss intensile strength.

DESCRIPTION OF THE DRAWINGS dancer arm 9, into preheater 10, wherein itis passed back and forth across rollers 11.

The temperature of the billet is from ambient temperature to 15C. belowthe crystalline melting point of .the particular polyethylene beingroll-oriented. Al-

though the billet can be roll oriented at room temperature the operationis performed more smoothly and with a substantial reduction in powerconsumption when an elevated temperature is used. It should be furthernoted that even though water is preferably used in the quench bathbecause of its ready availability and high specific heat, the billet ispreferably in anhydrous condition as it is fed into the orienting rolls.This is be cause the heat developed in the orientation rolls by therearrangement of the polyethylene molecules in the billet may causevaporization of any water or other low boiling liquid present in thebillet, and thereby, create voids or other flaws in the final strapping.The preheated billet is thenfed through one or more pairs of orientingrolls 12 and 13 and is drawn under tension out of the orienting rolls bymeans of tension rolls l4 and 15, and skew roll 16 used to enablemultiple wraps whereupon it is passed into hot oil stretching bath 17 inwhich it passes over roll 18 down through the hot oil around roll 19 andup and out of the hot oil bath over roll 20 and passed between tensionrolls 2.1 and 22 and around skew roll 23 to. enable multiple wrapsaround the tension rolls. The strapping is then passed through heatconditioner 24 equipped with exhaust 25, through wash tank 26 andfinally is taken up onto spool 27.

The orienting rolls preferably are of tongue and groove construction asshown in U. S. Pat. No. 3,354,023 referred to above. It is to beunderstood that while two pairs of orienting rolls are shown in FIG. 1,any desired number of rolls may be used. The function of the flanges onthe tongue and groove rolls is to assist in controlling the width of.the oriented strapping by controlling the size of opening defined bythe rolls. The amount of tension on the strapping which is imposed bythe speed of tension rolls 14, 15, and 16 relative to the speed rolls 12and 13, controls the amount of decrease in width the strapping undergoesafter leaving orienting rolls 12 and 13. The strappings of thisinvention are dis tinguished from films in that they have a highuniaxial orientation. The amount of stretch or necking down of thestrapping on leaving the orienting rolls must be accurately controlledsince the width of the final strapping is preferably within 10.005 inchof the width being sought or the strapping cannot readily be fastenedwith commercially available fasteners. These fasteners generally areheavy gauge metal seals or clips which fit around the strapping jointand are crimped with a machine similar to that commercially used to joinsteel strapping, such as those illustrated in U. S. Pat. No. 3,028,281,except preferably with straight sides or edges. Clips or seals requirewidth tolerances. In order to obtain a uniform rectangular billet it isnecessary to have the cross-section near the corners of the extrusiondie somewhat oversize. By using a die of this shape the tendency of theextrudate towards becoming round is overcome and a billet of trulyrectangular cross-section can be obtained. If a rectangularly shaped dieopening is used, the billet will have a nearly oval cross-section, andexcessive cross orientation will be imposed by the orienting rolls,thereby lowering the amountof length deformation which can be imposed onthe strapping which in turn lowers its ultimate strength and usefulness.

The polyethylene as used herein may be blended with up to 25 percent byweight as based on the total composition of an addition polymer such aslow density polyethylene, ethylene/vinyl acetate copolymer,ethylene/methacrylic acid copolymer, ethylene/vinyl acetate/methacrylicacid terpolymer, isotactic polypropylene, or moldable styrene/butadienerubber. Blending with 5-20 percent by weight as based on the totalcomposition of ethylene/vinyl acetate copolymer containing from 70 to 80percent ethylene and from 20 to 30 percent vinyl acetate is particularlyhelpful in improving resistance to splitting. The addition of isotacticpolypropylene tends to improve surface hardness. Pigments are readilyadded as a concentrate dispersed in low density polyethylene.

The melting behavior of the strapping of the present invention providesan indication of the difference in structure between the strapping ofthe present invention and prior art strappings. The melting behavior ofvarious high density polyethylene strappings is studied by differentialthermal analysis using a thermal analyzer using various heating rates offrom 0.5 to

4 ditional information. A heating rate of l0C./minute when used onsamples of strapping which have been oriented by rolling and airstretching alone exhibit a single melting peak near 137C. However,samples prepared in accordance with the present invention using arolling, air stretch and final hot oil stretch to high deformationratios when examined using a heating ratio of l0C./minute have multiplepeaks with a major peak at from 140 to 145C. It is believed that thishigher melting peak reflects the partial unfolding of lamellar crystalsto form extended chain crystals. Very slow heating rates would permitthese extended chain crystals to refold before melting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In each of the followingexamples the strapping was made on the apparatus above described usingone or two pairs of three-inch diameter tongue and groove roll orientingrolls as at 12 and 13, take-off rolls consisting of one 4 inch diameterrubber roll as at 14, one 4 inch diameter steel roll as at 15 and oneskew roll as at 16 to permit about 15 wraps, a vertical hot oil bath asat 17 and another set consisting of a 4 inch diameter rubber roll as at22, 4 inch diameter steel roll as at 21 and a skew roll as at 23 to drawthe strapping in the hot oil bath. The melt index of the polyethylenebeing used is determined in accordance with A.S.T.M. D-1238 using atemperature of 190C. and a load of 2160 g. on the melt indexer. Thetotal deformation ratio is the weight per unit length of the unorientedbillet divided by that of the oriented product. The stretch ratio in air(air str.) is the linear speed of the take-off rolls 14, 15 and 16divided by the linear speed of the final rolling stage 12 and 13. Theoil stretch (Oil str.) ratio is the ratio of the linear speeds of thetension rolls 21, 22 and 23 divided by the linear speed of rolls 14, 15and 16.

In Examples 4 5 7, 8, 9 andIO the polyethylene contained 1 percent andin Example 1 l, 3 percent ofa pigment concentrate consisting of percenttitanium dioxide, 0.75 percent aluminum stearate, and 24.25 percent of apolyethylene having a melt index of 3.5 and a density of 0.923.g./cc. InExample 8, 5 percent; in Example 9, 10 percent; and in Examples 10 and11, 20.

TABLE ij-Eiam ies Properties of final strap Poly- Defamation X-ray DTAethylene ratios Ulti- Transorien major Oil Tensile mate Initial versetation melt Melt No. 1 Melt Dcn- Air Oil temp. Thick Width str. elong.draw rolling Example index sity Total str. str. ('C.) (mils) (in.)(p.s.i.) ratio stages lO0C./rninute. The slowest heating rate of 0.5C./minute apparently permits recrystallization and a single melting peakof l34l37C. is observed for all high density polyethylene strappings.High heating rates give evidence of serious thermal lags and do not giveany adpercent by weight as based on the'total composition of anethylene/vinyl acetate copolymer containing 25 percent by weight ofvinyl acetate and 75 percent by weight of ethylene and having a meltindex of 2.0 i 0.4 was incorporated in the polyethylene from which thestrapping was formed. The column headed DTA is the differential thermalanalysis as measured at l0C./minute and the value reported is that ofthe largest peak. The quench bath was water maintained at temperaturesof from ambient to 50C. in all cases. The tensile strength and modulusdata were obtained in a conventional test machine equipped with slottedmounting rolls with a one-inch per minute loading rate and a five inchseparation between rolls. The test results are all based on the originaldimensions of the strapping.

The term uniplanar, axial orientation employed in defining the productof this invention may be fully understood from the following discussion.

Axial, planar, and uniplanar axial indicate different types of crystalorientation in high polymeric materials. Axial orientation means that agiven crystal axis (frequently the polymer chain axis) is parallel to amacroscopic axis (e.g., the machine direction in an extruded object).-For example, prior art materials which had been drawn in only onedirection (e.g., fibers or one-way stretched films) generally exhibit anappreciable degree of axial orientation in which the polymer chain axesare aligned parallel to the stretched direction. Planar" orientationmeans that a given crystal axis is parallel to a macroscopic levelplane. Conventional two-way stretched films for example generallyexhibit a degree of planar orientation in that the molecular chain axeslie approximately parallel to the surface of the film'although said axesare arranged at random within this plane. Uniplanar axial orientationmeans a given crystal axis is parallel to a macroscopic axis and a givencrystal plane is parallel to a macroscopic plane. In the rolled,extruded shapes discussed here the molecular chain axis is generally inthe direction of rolling and a certain crystal plane is parallel to therolled surface. As used here the terms axial, planar, and uniplanaraxial orientation refer not only to perfectalignmentof the typesdiscussed but also to structures in which there is a preferredorientation even though there may be some angular distributions aboutthe preferred orientation. Roll-oriented polymers generally exhibituniplanar, axial orientation."

X-ray diffraction furnishes a convenient technique for observing thetype of orientation in the ogjects of this invention. A sample ismounted on an instrument such-as a Single Crystal Orienter which has theability to rotate the sample in the X-ray beam about two mutuallyperpendicular axes. Since a crystalline material will diffract X-raysonly when the X-ray beam, the detector, and suitable crystalline planeswithin the sample are arranged in the manner described by Braggs Law, itis possible to determine the crystal orientation within the sample bystudying the variation in the intensity of the diffracted X-rays as thesample is rotated. This intensity will pass through a maximum as theangular orientation of the sample reaches a value corresponding to themost populous orientation of the crystals within the sample. The breadthof the distribution of crystal orientations may be characterized by thewidth of a plot of X-ray intensity vs. the angular orientation of thesample at an intensity value equal to one-half of the peak maximum.Further aspects of the definition of the types of orientation and oftechniques for determining the distribution of crystal orientation insynthetic polymers are described in a paper by C. J. Heffelfinger and R.L. Burton in the Journal of Polymer Science, Volume 47, pages 289-306(1960).

In an extruded, rolled shape made from high density polyethylene, theuniplanar axial orientation is such that the polymer chains tend to bein the direction of rolling and either the or the 1 10) crystal planestend to be parallel to the rolled surface. The angular width at theone-half maximum corresponding to the tilting of the polymer chains fromthe roll direction toward the transverse direction is less than 7 andpreferably from 45 to 6. These angles correspond to those obtained foruniplanar axial orientation in high density polyethylene which has beenroll and stretched to increase its length at least ten-fold andpreferably from 11 to 13.5 fold.

It is well known in the art that controlled deformation of a crystallinepolymer results in an improvement in the physical properties of thepolymer'in the direction of deformation. This is most highly developedin the case of fiber and filaments where very marked improvement intensile strength and modulus with an axial orientation is obtained bycold drawing of the extruded fiber or filament. Attempts to obtainequivalent improvement in physical properties in more massivepolyethylene shapes with triaxial symmetry such as tapes, straps,sheets, angles, tees, and the like have not succeeded in producingstrengths above about 40,000 psi. as is illustrated by control Examples1 and 6 which do not utilize the hot stretch step.

I claim:

1. A uniformly orientedstrap formed of at least 75 weight percentpolyethylene having a density of from 0.95 to 0.967 g per cc, a meltindex of from 0.2 to 1.0, said strap having a major melt peak asmeasured by differential thermal analysis at a rate of 10C. per minuteof to C, and an angular width at the one-half maximum as determined byX-ray corresponding to the tilting of the polymer chains from thelongitudinal direction toward the transverse direction of from 45 to 2.The strap of claim 1 wherein the resin consists essentially ofpolyethylene having a melt index between 0.4 and 0.9. g

3. The strapping of claim 1 wherein the resin consists essentially ofpolyethylene and a copolymer of ethylene and vinyl acetate having a meltindex from 1.0 to 3.0 and containing from 70 to 80 weight per centethylene and from 20 to 30 weight percent vinyl acetate.

2. The strap of claim 1 wherein the resin consists essentially ofpolyethylene having a melt index between 0.4 and 0.9.
 3. The strappingof claim 1 wherein the resin consists essentially of polyethylene and acopolymer of ethylene and vinyl acetate having a melt index from 1.0 to3.0 and containing from 70 to 80 weight per cent ethylene and from 20 to30 weight percent vinyl acetate.